1. Field of the Invention
The present invention relates to a layered catalyst composite useful for reducing contaminants in exhaust gas streams, especially gaseous streams containing sulfur oxide contaminants. More specifically, the present invention is concerned with improved catalysts of the type generally referred to as “three-way conversion” catalysts. The layered catalysts trap sulfur oxide contaminants which tend to poison three-way conversion catalysts used to abate other pollutants in the stream. The layered catalyst composites of the present invention have a sulfur oxide absorbing layer before or above a nitrogen oxide absorbing layer. The sulfur oxide absorbing layer selectively and reversibly absorbs sulfur oxides over nitrogen oxides and alleviates sulfur oxide poisoning of the nitrogen oxide trap.
2. Related Art
Three-way conversion catalysts (“TWC”) have utility in a number of fields including the abatement of nitrogen oxides (“NOx”), carbon monoxide (“CO”), and hydrocarbon (“HC”) pollutants from internal combustion engines, such as automobile and other gasoline-fueled engines. Three-way conversion catalysts are polyfunctional because they have the ability to substantially simultaneously catalyze the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides. Emissions standards for nitrogen oxides, carbon monoxide, and unburned hydrocarbon contaminants have been set by various government agencies and must be met by new automobiles. In order to meet such standards, catalytic converters containing a TWC catalyst are located in the exhaust gas line of internal combustion engines. The catalysts promote the oxidation by oxygen in the exhaust gas of the unburned hydrocarbons and carbon monoxide and the reduction of nitrogen oxides to nitrogen. For example, it is known to treat the exhaust of engines with a catalyst/NOx sorbent which stores NOx during periods of lean (oxygen-rich) operation, and releases the stored NOx during the rich (relatively fuel-rich) periods of operation. During periods of rich operation, the catalyst component of the catalyst/NOx sorbent promotes the reduction of NOx to nitrogen by reaction of NOx (including NOx released from the NOx sorbent) with HC, CO, and/or hydrogen present in the exhaust.
TWC catalysts exhibiting good activity and long life comprise one or more platinum group metals, e.g., platinum, palladium, rhodium, ruthenium, and iridium. These catalysts are employed with a high surface area, refractory oxide support such as a high surface area alumina coating. The support is carried on a suitable carrier or substrate such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure, or refractory particles such as spheres or short, extruded segments of a suitable refractory material. The supported catalyst is generally used with a NOx storage (sorbent) component including alkaline earth metal oxides, such as oxides of Ca, Sr and Ba, alkali metal oxides such as oxides of K, Na, Li and Cs, and rare earth metal oxides such as oxides of Ce, La, Pr and Nd, see U.S. Pat. No. 5,473,887.
Sulfur oxide (“SOx”) contaminants present in an exhaust gaseous stream tend to poison and thereby inactivate TWC catalysts. SOX is a particular problem because it is generated by the oxidation of sulfur compound impurities often found in gasoline and diesel fuel. TWC catalysts employing NOX storage components tend to suffer from loss of long-term activity because of SOx poisoning of the NOx traps. NOx trap components also trap SOx and form very stable sulfates which require extreme conditions and a high fuel penalty to regenerate the trapping capacity of the NOx storage component. A guard or filter (e.g., alumina) may be placed before the TWC catalyst to protect the catalyst from SOx but these guards or filters often become saturated with SoX. Without valves, these guards require artificial engine cycles to desorb SOx by creating extended rich A/F period at elevated temperature. However, the SOx released under these conditions normally caused high H2S emission with unpleasant odor and to some extent poison the downstream NOx absorber.
High surface refractory metal oxides are often employed as a support for many of the catalytic components. For example, high surface area alumina materials, also referred to as “gamma alumina” or “activated alumina” typically exhibit a BET (Brunauer, Emmett, and Teller) surface area in excess of 60 square meters per gram (“m2/g”), and often up to about 200 m2/g or more. Such activated alumina is usually a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of eta, kappa and theta alumina phases. Refractory metal oxides other than activated alumina may be utilized as a support for at least some of the catalytic components in a given catalyst. For example, bulk ceria, zirconia, alpha alumina and other materials are known for such use. Although many of these materials have a lower BET surface area than activated alumina, that disadvantage tends to be offset by the greater durability of the resulting catalyst.
Exhaust gas temperatures can reach 1000° C. in a moving vehicle and such elevated temperatures can cause activated alumina, or other support material, to undergo thermal degradation with accompanying volume shrinkage especially in the presence of steam. During this degradation, the catalytic metal becomes occluded in the shrunken support medium with a loss of exposed catalyst surface area and a corresponding decrease in catalytic activity. U.S. Pat. No. 4,171,288 discloses a method to stabilize alumina supports against such thermal degradation by the use of materials such as zirconia, titania, alkaline earth metal oxides such as baria, calcia, or strontia, or rare earth metal oxides such as ceria, lanthana, and mixtures of two or more rare earth metal oxides.
U.S. Pat. Nos. 4,714,694, 4.727,052, and 4,708,946 disclose the use of bulk cerium oxide (ceria) to provide a refractory oxide support for platinum group metals other than rhodium. Highly dispersed, small crystallites of platinum on the ceria particles may be formed and stabilized by impregnation with a solution of an aluminum compound followed by calcination.
U.S. Pat. No. 3,993,572 discloses catalysts for promoting selective oxidation and reduction reactions. The catalyst contains platinum group metal, rare earth metal (ceria) and alumina components which may be supported on a relatively inert carrier such as a honeycomb.
U.S. Pat. No. 4,714,694 discloses a method of making a material which includes impregnating bulk ceria or a bulk ceria precursor with an aluminum compound and calcining the impregnated ceria to provide an aluminum stabilized ceria.
U.S. Pat. No. 4,808,564 discloses a catalyst for the purification of exhaust gases having improved durability which comprises a support substrate, a catalyst carrier layer formed on the support substrate and catalyst ingredients carried on the catalyst carrier layer. The catalyst carrier layer comprises oxides of lanthanum and cerium in which the molar fraction of lanthanum atoms to total rare earth atoms is 0.05 to 0.20 and the ratio of the number of the total rare earth atoms to the number of aluminum atoms is 0.05 to 0.25.
U.S. Pat. No. 4,367,162 discloses a three-way catalyst system which comprises a carrier having a substructure of refractory material in the form of a honeycomb structure and a porous layer of a powder formed on the surface thereof selected from the group consisting of a powder of zirconium oxide and a mixed powder of zirconium oxide powder with at least powder selected from the group consisting of alumina, alumina-magnesia spinel and cerium oxide, and a catalyst ingredient supported thereon consisting of cerium oxide and a metal selected from the group consisting of platinum, palladium, and mixtures thereof.
U.S. Pat. No. 4,438,219 discloses an alumina catalyst, stable at high temperatures, for use on a substrate. The stabilizing material is derived from barium, silicon, rare earth metals, alkali and alkaline earth metals, boron, thorium, hafnium, and zirconium. Barium oxide, silicon dioxide, and rare earth oxides including lanthanum, cerium, praseodymium, and neodymium are preferred. Contacting the stabilizing material with a calcined alumina film permits the calcined alumina film to retain a high surface area at higher temperatures.
U.S. Pat. Nos. 4,476,246, 4,591,578 and 4,591,580 disclose three-way catalyst compositions comprising alumina, ceria, an alkali metal oxide promoter, and Noble metals. U.S. Pat. Nos. 3,993,572 and 4,157,316 describe attempts to improve the catalyst efficiency of Pt/Rh based TWC systems by incorporating a variety of metal oxides, e.g., rare earth metal oxides such as ceria and base metal oxides such as nickel oxides. U.S. Pat. No. 4,591,518 discloses a catalyst comprising an alumina support with catalytic components consisting essentially of a lanthana component, ceria, an alkali metal oxide, and a platinum group metal. U.S. Pat. No. 4,591,580 discloses an alumina supported platinum group metal catalyst modified to include support stabilization by lanthana or lanthana rich rare earth oxides, double promotion by ceria and alkali metal oxides and optionally nickel oxide.
U.S. Pat. No. 4,624,940 discloses palladium containing catalyst compositions useful for high temperature applications. The combination of lanthanum and barium is found to provide a superior hydrothermal stabilization of alumina which supports the catalytic component, palladium. Thus, the palladium metal expulsion from the alumina due to phase transformation to encounter drastic sintering upon high temperature exposure is avoided. The use of particulate bulk metal oxide enhances catalytic activities. The bulk metal oxide consists of primarily ceria containing and/or ceria-zirconia containing particles. These particulate bulk metal oxides do not readily react with the stabilized alumina particles, thus, provide the catalytically promoting effect.
U.S. Pat. No. 4,780,447 discloses a catalyst capable of controlling HC, CO and NOx as well as H2S in emissions from the tailpipe of catalytic converter equipped automobiles. The use of nickel oxides and/or iron oxides is known as a H2S gettering of compound.
U.S. Pat. No. 4,294,726 discloses a TWC catalyst composition containing platinum and rhodium obtained by impregnating a gamma alumina carrier material with an aqueous solution of cerium, zirconium and iron salts or mixing the alumina with oxides of, respectively, cerium, zirconium and iron, and then calcining the material at 500° C. to 700° C. in air after which the material is impregnated with an aqueous solution of a salt of platinum and a salt of rhodium dried and subsequently treated in a hydrogen-containing gas at a temperature of 250° C.-650° C. The alumina may be thermally stabilized with calcium, strontium, magnesium or barium compounds. The ceria-zirconia-iron oxide treatment is followed by impregnating the treated carrier material with aqueous salts of platinum and rhodium and then calcining the impregnated material.
U.S. Pat. No. 4,965,243 discloses a method to improve the thermal stability of a TWC catalyst containing precious metals by incorporating a barium compound and a zirconium compound together with ceria and alumina to form a catalytic moiety to enhance stability of the alumina washcoat upon exposure to high temperature.
J01210032 and AU-615721 disclose a catalytic composition comprising palladium, rhodium, active alumina, a cerium compound, a strontium compound and a zirconium compound. These patents suggests the utility of alkaline earth metals in combination with ceria, zirconia to form a thermally stable alumina supported palladium containing washcoat.
U.S. Pat. No. 4,504,598 discloses a process for producing a high temperature resistant TWC catalyst. The process includes forming an aqueous slurry of particles of gamma or activated alumina and impregnating the alumina with soluble salts of selected metals including cerium, zirconium, at least one of iron and nickel and at least one of platinum, palladium and rhodium and, optionally, at least one of neodymium, lanthanum, and praseodymium. The impregnated alumina is calcined at 600° C. and then dispersed in water to prepare a slurry which is coated on a honeycomb carrier and dried to obtain a finished catalyst.
U.S. Pat. Nos. 3,787,560, 3,676,370, 3,552,913, 3,545,917, 3,524,721 and 3,899,444 disclose the use of neodymium oxide for use in reducing nitric oxide in exhaust gases of internal combustion engines. U.S. Pat. No. 3,899,444 in particular discloses that rare earth metals of the lanthanide series are useful with alumina to form an activated stabilized catalyst support when calcined at elevated temperatures. Such rare earth metals are disclosed to include lanthanum, ceria, cerium, praseodymium, neodymium and others.
U.S. Pat. No. 5,792,436 discloses a method for removing nitrogen oxides, sulfur oxides, and phosphorus oxides from a lean gaseous stream. The method comprises (a) passing the gaseous stream through a catalyzed trap comprising a regenerable sorbent material and an oxidation catalyst and sorbing the sorbable components into the sorbent material, (b) introducing a combustible component into the gaseous stream upstream of the catalyzed trap member and combusting the combustible component in the presence of the oxidation catalyst to thermally desorb the sorbable component from the sorbent material, and (c) passing the sorbable component-depleted stream to a catalytic treatment zone for the abatement of the pollutants and by-passing the sorbable component-enriched stream around the catalytic treatment zone.
TWC catalyst systems comprising a carrier and two or more layers of refractory oxide are disclosed. Japanese Patent Publication No. 145381/1975 discloses a catalyst-supported structure for purifying exhaust gases comprising a thermally insulating ceramic carrier and at least two layers of catalyst containing alumina or zirconia, the catalysts containing alumina or zirconia layers being different from each other.
Japanese Patent Publication No. 105240/1982 discloses a catalyst for purifying exhaust gases containing at least two carrier layers of a refractory metal oxide, each containing a different platinum-group metal. A layer of a refractory metal oxide free from the platinum-group metal is positioned between the carrier layers and/or on the outside of these carrier layers.
Japanese Patent Publication No. 52530/1984 discloses a catalyst having a first porous carrier layer composed of an inorganic substrate and a heat-resistant Noble metal-type catalyst deposited on the surface of the substrate and a second heat-resistant non-porous granular carrier layer having deposited thereon a Noble metal-type catalyst. The second carrier layer is formed on the surface of the first carrier layer and has resistance to the catalyst poison.
Japanese Patent Publication No. 127649/1984 discloses a catalyst for purifying exhaust gases comprising an inorganic carrier substrate such as cordierite, an alumina layer formed on the surface of the substrate and having deposited thereon a rare earth metal, such as lanthanum and cerium, and platinum or palladium, and a second layer formed on the first alumina-based layer and having deposited thereon a base metal such as iron or nickel and a rare earth metal such as lanthanum or rhodium.
Japanese Patent Publication No. 19036/1985 discloses a catalyst for purifying exhaust gases having an enhanced ability to remove carbon monoxide at low temperatures. The catalyst comprises a substrate composed of cordierite and two layers of active alumina laminated to the surface of the substrate. The lower alumina layer contains platinum or vanadium deposited thereon, and the upper alumina layer contains rhodium and platinum, or rhodium and palladium, deposited thereon.
Japanese Patent Publication No. 31828/1985 discloses a catalyst for purifying exhaust gases comprising a honeycomb carrier and a Noble metal having a catalytic action for purifying exhaust gases. The carrier is covered with an inside and an outside alumina layer, the inside layer having more Noble metal adsorbed thereon than the outside layer.
Japanese Patent Publication No. 232253/1985 discloses a monolithic catalyst for purifying exhaust gases in the shape of a pillar and comprising a number of cells disposed from an exhaust gas inlet side toward an exhaust gas outlet side. An alumina layer is formed on the inner wall surface of each of the cells and catalyst ingredients are deposited on the alumina layer. The alumina layer consists of a first alumina layer on the inside and a second alumina layer on the surface side, the first alumina layer having palladium and neodymium, and the second alumina layer having platinum and rhodium.
Japanese Kokai 71538/87 discloses a catalyst layer supported on a catalyst carrier and containing one catalyst component selected from the group consisting of platinum, palladium and rhodium. An alumina coat layer is provided on the catalyst layer. The coat layer contains one oxide selected from the group consisting of cerium oxide, nickel oxide, molybdenum oxide, iron oxide and at least one oxide of lanthanum and neodymium (1-10% by wt.).
U.S. Pat. Nos. 3,956,188 and 4,021,185 disclose a catalyst composition having (a) a catalytically active, calcined composite of alumina, a rare earth metal oxide and a metal oxide selected from the group consisting of an oxide of chromium, tungsten, a group IVB metal and mixtures thereof and (b) a catalytically effective amount of a platinum group metal added thereto after calcination of the composite. The rare earth metals include cerium, lanthanum and neodymium.
U.S. Pat. No. 4,806,519, discloses a two layer catalyst structure having alumina, ceria and platinum on the inner layer and aluminum, zirconium and rhodium on the outer layer.
JP-88-240947 discloses a catalyst composite which includes an alumina layer containing ceria, ceria-doped alumina and at least one component selected from the group of platinum, palladium and rhodium. A second layer contains lanthanum-doped alumina, praseodymium-stabilized zirconium, and lanthanum oxide and at least one component selected from the group of palladium and rhodium. The two layers are placed on a catalyst carrier separately to form a catalyst for exhaust gas purification.
Japanese Patent J-63-205141-A discloses a layered automotive catalyst in which the bottom layer comprises platinum or platinum and rhodium dispersed on an alumina support containing rare earth oxides, and a top coat which comprises palladium and rhodium dispersed on a support comprising alumina, zirconia and rare earth oxides.
Japanese Patent J-63-077544-A discloses a layered automotive catalyst having a first layer comprising palladium dispersed on a support comprising alumina, lanthana and other rare earth oxides and a second coat comprising rhodium dispersed on a support comprising alumina, zirconia, lanthana and rare earth oxides.
Japanese Patent J-63-007895-A discloses an exhaust gas catalyst comprising two catalytic components. One component comprises platinum dispersed on a refractory inorganic oxide support and a second component comprises palladium and rhodium dispersed on a refractory inorganic oxide support.
U.S. Pat. No. 4,587,231 discloses a method of producing a monolithic three-way catalyst for the purification of exhaust gases. A mixed oxide coating is applied to a monolithic carrier by treating the carrier with a coating slip in which an active alumina powder containing cerium oxide is dispersed together with a ceria powder and then baking the treated carrier. Platinum, rhodium and/or palladium are then deposited on the oxide coating by a thermal decomposition. Optionally, a zirconia powder may be added to the coating slip.
U.S. Pat. No. 4,134,860 relates to catalyst compositions that can contain platinum group metals, base metals, rare earth metals and refractory supports. The composition can be deposited on a relatively inert carrier such as a honeycomb. U.S. Pat. No. 4,923,842 discloses a catalytic composition for treating exhaust gases comprising a first support having dispersed thereon at least one oxygen storage component and at least one Noble metal component, and having dispersed immediately thereon an overlayer comprising lanthanum oxide and optionally a second support. The layer of catalyst is separate from the lanthanum oxide. The Nobel metal can include platinum, palladium, rhodium, ruthenium and iridium. The oxygen storage component can include the oxide of a metal from the group consisting of iron, nickel, cobalt and the rare earths. Illustrative of these are cerium, lanthanum, neodymium, praseodymium, etc.
U.S. Pat. No. 5,057,483 discloses a catalyst composition disposed in two discrete coats on a carrier. The first coat includes a stabilized alumina support on which a first platinum catalytic component and bulk ceria is dispersed, a bulk iron oxide, a metal oxide such as bulk nickel oxide (which is effective for the suppression of hydrogen sulfide emissions), and one or both of baria and zirconia dispersed throughout the first coat as a thermal stabilizer. The second coat, which may comprise a top coat overlying the first coat, contains a co-formed (e.g., co-precipitated) rare earth oxide-zirconia support on which a first rhodium catalytic component is dispersed, and a second activated alumina support having a second platinum catalytic component dispersed thereon. The second coat may also include a second rhodium catalytic component, and optionally, a third platinum catalytic component, dispersed as an activated alumina support.
U.S. Pat. No. 5,472,673 discloses an exhaust gas purification device for an engine. The device comprises an engine, an exhaust passage, an NOx absorbent, and a sulphur trapping means. The exhaust passage extends from an upstream end which receives exhaust gas from the engine to a downstream end from which exhaust gas is released. The NOx absorbent is arranged in the exhaust passage wherein the NOx absorbent absorbs NOx contained in the exhaust gas when a concatenation of oxygen in the exhaust gas flowing into the NOx absorbent is above a predetermined oxygen concentration. The NOx absorbent releases the absorbed NOx when the concentration of oxygen in the exhaust gas flowing into the NOx absorbent is lower than the predetermined oxygen concentration. The sulphur trapping means is arranged in the exhaust passage upstream of the NOx absorbent for trapping SOx contained in the exhaust gas wherein the trapped SOx is not released from the sulphur trapping means when the concentration of oxygen in the exhaust gas flowing into the sulphur trapping means is lower than the predetermined oxygen concentration so that SOx is prevented from reaching and being absorbed into the NOx absorbent.
U.S. Pat. No. 5,687,565 discloses a method for reducing the concentration of carbon monoxide, organic compounds and sulfur oxides in an exhaust gas from an internal combustion engine. The method comprises (a) contacting the exhaust gas with a sulfur oxide absorbent in a first contacting zone and absorbing with the sulfur oxide absorbent at least a portion of the sulfur oxides in the exhaust gas wherein the sulfur oxide absorption is substantially irreversible at temperatures which are less than or equal to that of the exhaust gas; (b) contacting the effluent gas from the first contacting zone with a catalyst in a second contacting zone and catalyzing the conversion of at least a portion of the carbon monoxide and organic compounds in the effluent gas from the first contacting zone to innocuous products; and (c) transferring heat from the exhaust gas to the second contacting zone by indirect heat exchange.
U.S. Pat. No. 5,687,565 discloses a system for exhaust gas purification disposed in an exhaust pipe of an internal combustion engine. The system comprises a catalyst composition giving an excellent light-off performance at low temperatures which comprises a precious metal and a substance having at least one of an electron donatability and a nitrogen dioxide absorbability and releasability, and optionally an adsorbent having hydrocarbon adsorbability.
WO92/09848 discloses a combustion catalyst comprising palladium and optionally a Group 1B or VIII noble metal which may be placed on a support comprising zirconium. The combustion catalyst may be graded to have a higher activity portion at the leading edge of the catalyst structure. The invention includes a partial combustion process in which the fuel is partially combusted using that catalyst. The catalyst structure is stable in operation, has a comparatively low operating temperature, has a low “light off” temperature, and is not susceptible to temperature “runaway”. The combustion gas produced by the catalytic process may be at a temperature below the autocombustive temperature, may be used at that temperature, or fed to other combustive stages for further use in a gas turbine, furnace, or boiler.
The conventional catalysts described above employing NOx storage components have the disadvantage under practical applications of suffering from long-term activity loss because of SOx poisoning of the NOx traps. The NOx trap components employed in the catalysts tend to trap SOx and form very stable sulfates which require extreme conditions and extract a high fuel penalty to regenerate the trapping capacity of the NOx storage component. Accordingly, it is a continuing goal to develop a three-way catalyst system which can reversibly trap SOx present in the gaseous stream and thereby prevent SOx sulfur oxide poisoning of the NOx trap.